A centrally controlled fire alarm system can comprise a "central control unit" (or CCU) for monitoring different groups of fire detecting devices located in different parts of a building. Each group of fire detecting devices can be connected across a common pair of supply lines which are linked back to the CCU which normally applies say 12 volts to the lines to operate the detecting devices. Alarm devices, which are triggered by a higher voltage, can be connected across the same pair of common supply lines and the CCU can respond to a "fire detection signal" to apply say 24 volts to the lines so a to cause the alarm devices to give "alarm signals". This system avoids an excessive amount of wiring compared with a system where detecting devices and alarm devices are connected to respective dedicated supply lines. When such a system is in a standby condition, the supply voltage is below a threshold of, for example, 18 volts, and each fire detection device has a high line impedance, thereby drawing little or no current from the supply. On detecting a fire, the fire detecting device produces a detection signal by changing from a high line impedance to a low line impedance, for example, by switching a known resistance across the supply lines. The detection signal is detected by a control unit which then applies the higher voltage (24 v) to the lines. As the threshold voltage is then exceeded, the alarm devices are activated to produce warning signals. Such a system is disclosed in our copending UK application no. 9808094.8 to which reference may be made for further details. Alternatively or additionally, an alarm device may produce a warning signal when the polarity of the supply is reversed.
Where a fire alarm system is used to protect life or property, national regulations or codes of practice usually require the supply lines connecting the detection and alarm devices to the CCU to be monitored for an open circuit fault condition and for such fault condition to be indicated and a fault warning given at the CCU. Some regulations also require the supply lines to be monitored for a short circuit fault and for such fault to be indicted at the CCU. By connecting an "end of line device", for example a resistor, across the remote end of the supply lines so as to establish a known current in the supply lines, the CCU can indicate a fault when the current in the supply lines is abnormally low or high.
Where, to facilitate servicing, fire detectors comprise a detector head which is easily detachable from a mounting base fitted with terminals for connection to supply lines, regulations normally require that the CCU indicates a fault condition when a detector head is removed. This requirement may be met by causing the detector head to open circuit one of the supply lines when it is detached from the mounting base, thereby causing the CCU to indicate a fault condition in response to a supply line open circuit.
FIG. 1 shows a typical wiring arrangement of supply lines from a CCU to fire detector mounting bases (8) and alarm devices. When a detector head is attached to a mounting base a conducting path is completed between terminals L1 IN and L1 OUT via electrically connected corresponding terminals on the detector head that engage with terminals L1 IN and L1 OUT. A further terminal on the detecting head engages with mounting base terminal L2 thereby completing the supply connection to the detector head.
The arrangement shown in FIG. 1 has the disadvantage that removal of a detector head open-circuits a supply line to detection devices and alarm devices further away from the CCU than the detector head removed, thereby rendering those devices inoperative and parts of the building unprotected. Under some regulations this arrangement is not permitted, particularly if an alarm device or a manually operated detection device (manual call point) is rendered inoperative by the removal of a detector head. This problem can be overcome by connecting all alarm devices and manual call points closer to the CCU than any detecting device with a detachable detector head. However this approach uses excessive wiring if the optimum positioning of detection devices, manual call points and alarm devices is not to be compromised.
FIG. 2 shows an alternative wiring arrangement in which mounting base terminals L1 IN and L1 OUT are permanently connected by a diode (D1). When the detector head is attached to the base (8) the diode is short-circuited by the conducting path through the detector head linking L1 IN and L1 OUT. When the detector head is removed the diode maintains a conducting path between terminals L1 IN and L1 OUT, the diode being connected so as to maintain the supply of power to detection devices and alarm devices beyond the point where the detector head was removed. Because an open circuit is not introduced by removal of a detector head, means must be provided for the presence of a diode in the supply line to be detected in order for the removal of a detector head to be detected and a fault indicated at the CCU.
Monitoring for detector removal using the diode means described above has limitations. Firstly, the removal of a detector head produces a voltage drop, equal to the forward bias voltage of the diode D1, in the supply to detection devices and alarm devices beyond the point at which the head was removed. As more heads are removed the voltage supplied to devices near the remote end of the supply lines progressively decreases. For example, if 20 heads are removed, the decrease would be typically greater than 10 volts for general purpose silicon diodes, and over 5 volts for Schottky diodes. This limits the number of detector heads that can be removed without reducing the supply voltage below the minimum operating voltage of the alarm devices and the remaining detection devices, including manual call points. A second limitation is that where detection devices and alarm devices are connected to the same pair of supply lines, both device types must be operated in the same polarity.
GB-A-2069205 discloses other means for detecting detector head removal without interrupting the supply to detection devices and alarm devices or introducing a diode in a supply line. FIG. 3 shows a typical wiring arrangement in which the lines between detection devices are continuous and not broken by the removal of a detector head (7). FIG. 3 also shows circuit means incorporated in the base (8) of each detection device. When the head (7) is attached to the base (8), the transistor T1 is turned off and the circuit presents a high line impedance. When the detector head is removed, the short circuit that existed between the base and emitter of T1, via head and base interconnecting terminals L1 IN and L1 OUT, is broken and T1 conducts and connects a load, a zener diode ZD1, across the line. The consequent reduction in line voltage is detected by the CCU and interpreted as a detector removed fault and indicated accordingly. In systems using the disclosed means, alarm devices are either connected to separate dedicated lines or to the same lines as the detection devices and activated by reversing the polarity of the supply. Activating alarm devices by increasing the supply voltage above a threshold voltage is not practical because a zener diode connected across supply lines at any device with a head removed prevents an increase in supply voltage.
GB-A-2313690 describes another such system. Each detection devices comprises a circuit arranged to periodically connect a load across the supply lines in order to produce a fault signal and a switch which activates the circuit when the head of the device is removed. A monitoring circuit in the CCU triggers a fault warning alarm when a fault signal is detected. This arrangement has the following disadvantages or limitations:
(1) The fault signal produced by the removal of a head (i.e. periodic application of a load), would increase the current drawn from the supply if the supply voltage was switched from a low voltage (12 volts) current limited supply (25 mA) to a higher voltage, higher current limited supply (e.g. 24 volts, 1 amp) for the purpose of activating voltage threshold controlled alarm devices. This would increase the capacity and cost of any standby battery supply for the system. This problem is exacerbated by the removal of a multiplicity of detector heads. PA1 (2) When a multiplicity of heads are removed the multiplicity of fault signals applied to the supply lines can cause perceptible interference on the output of some types of alarm devices. (For example, the alarm devices may thereby produce intermittent audible sounds). PA1 (3) When a multiplicity of heads are removed there is a possibility that their respective fault signals will be generated so that their loads are simultaneously connected across the lines causing undesirable high current surges to be drawn from the low impedance supply for the alarm devices. PA1 (4) When a multiplicity of heads are removed there is a possibility that fault signals applied to the lines by different devices will either overlap or be close together in time as to effectively extended the period for which a load is applied. The circuit means in the CCU monitoring for a detector removed fault signal must therefore be able to distinguish between extended signals and a fire detection signal thereby delaying detection of a fire detection signal. PA1 (5) Overlapping fault signals can reduce the supply voltage to a low level for a time sufficient to reset a fire detection signal produced by some types of detection devices. A fire detection signal produced by many types of detection device will be reset by a supply interruption of less than 20 milliseconds without the fire detection being reset.
These limitations are so significant that alarm devices may not be connected in parallel with detection devices. For instance, FIG. 1 of GB-A-2313690 indicates that the supply to alarm devices is provided by a separate pair of lines, even when detection and alarm devices are combined in one device.
Despite all of these various attempts in the past to solve the problem of detecting removal of a plurality of detector heads, no solution has been found which does not in some way have a serious limitation.